The illusory calm of Australia's deadly ocean currents

Rip currents are strong, narrow channels that pull the unwitting away from the shore and some move faster than an Olympic swimmer.

"The way they work on surf beaches, the white water - the breaking waves - bring water to the beach," said Rob Brander, a geomorphologist -someone who studies the way the ocean and the land interacts - at the University of New South Wales.

It piles up and has got to go out somehow and the vast majority of rips take that water out. It sometimes it looks like the safest place to swim because there are no waves breaking, and that's why so many people get in trouble. The rip is just like a river and it is taking you away."

The threat is highlighted by a simple, yet vivid experiment. At Tamarama in Sydney, one of Australia's most hazardous beaches, Dr Brander wades chest-high into the aqua blue water and pours in a cup full of harmless dye. Within a few seconds, the purple streak starts to expand and follows the rip out to sea. After a minute, the dark trail has travelled up to 100m (328ft).

From the sandstone headlands that overlook this idyllic stretch of sand, the dye arcs towards a ragged belt of rocks.

"Tamarama gets everybody. Every local here has a story about being caught in a rip, and tourists don't really know the power of our ocean," says Chris Chapman, a lifeguard with Waverley council, which patrols some of the most popular beaches in Australia.

'Dangerous rescues'

Rob Brander is a geomorphologist at the University of New South Wales

Tamarama lies to the south of its bigger and more boisterous cousin, Bondi. It is a small beach with powerful surf that routinely traps the unwary or careless. During the warmer months, about 100 swimmers are pulled from its rip currents each week. Lifesavers often risk their lives leaping from the cliffs into frothing seas to reach the stricken.

"They are big rescues and they are dangerous rescues for us. We try to stop people before they get into trouble," Mr Chapman said.

The Australian mainland is blessed with 11,000 beaches, but it is estimated that at any given time, there are 17,500 rips at work around the coast.

A recent survey showed that half of all Australians either do not know how to spot the dangerous currents or what to do if they find themselves entangled in one.

The most common rips lurk beneath dark green, calm-looking water between waves. While they remain misunderstood by many locals, tourists too have little knowledge of the potential dangers.

"I really haven't got a clue what a rip current is, but I'd hazard a guess that they are where waves meet. The whole beach looks really quite inviting," said Mark, a drama teacher from Britain.

"My first time I came to Sydney, my friend and I got caught by a current. We didn't quite know how to take the current. Fortunately for us it wasn't as violent as it could've been."

A survey by the University of New South Wales found that rip currents claim about 21 lives each year in Australia, more than the combined average annual toll for bushfires, cyclones, floods and shark attacks.

'Out of control'

Not many people are aware of the potential dangers of rip currents

There are countless near-misses by those who have managed to survive frightening encounters.

"It started to get scary. The only advice I could remember was to swim parallel with the beach. It didn't seem to work and I swam as hard as I could [but] ran out of breath," said Scott Clements, who was sucked out to sea while swimming with his partner.

He eventually fought his way back to the beach, only to find the drama was far from over.

"Once I got out I just about collapsed, except that was just the exact moment my girlfriend was getting towed out. I could see the look on her face, you know, terror," he said.

Mr Clements' girlfriend survived, and tragedy was avoided - but only just.

"People are out of control, they can't touch anything; they can't grab anything. So they try to swim back to the beach, invariably often against the rip, they get tired, they don't make it, then they panic," said Dr Brander.

The advice for those trapped by fast-moving channels is to try to stay calm, float with the current and signal for help.

The difference between life and death can come down to moments of clarity and composure in an energy-sapping maelstrom of churning water and mounting fear.